There are occasions where downhole devices such as packers or bridge plugs or cement shoes are milled out. Other times there is a tubing string or portion of a tubing string that needs to be cut so that subsequent operations can continue. Over time the design of such mills has evolved to address the need for greater speed and cutting efficiency. In the 1980s Baker Hughes came out with a line of mills known as Metal Muncher® as illustrated in U.S. Pat. Nos. 5,038,859 or 5,086,838; 4,796,709 and 5,456,312.
One example of this design is shown in FIGS. 1 and 2. The typical mill of this type had a body 10 with a central flow passage 12 that lead to a plurality of outlets 14 shown on the bottom face view of FIG. 2. A series of spaced apart vertical blades 16 had their leading face covered with a nested array of round inserts 18 made of a hardened cutting material such as tungsten carbide. These inserts were arranged in rows such as 20 and 22 and as one row would wear away with the blade that supported it the next row would take over the cutting task. Fluid such as drilling mud would be pumped through the outlets 14 located ahead of the inserts 18 on a given blade 16 in the direction of rotation. A matrix material 24 is deployed behind the blades 16 for structural support and for some limited cutting capability. The cuttings made by the inserts have to clear the outside edge of the mill and are carried off by the circulating fluid that also removes some of the heat generated from the milling operation.
Another prior mill design in three styles is illustrated in FIGS. 3-5. Here there are no blades and the matrix material with the crushed carbide particles in seen in a symmetrical array of pie shapes 26 about a center where there is no matrix or carbide particles. Each of the pie shapes has the identical formulation as the others. In some applications there are fluid outlets 28 or 30 illustrated to carry off cuttings and heat generated from the milling operation.
The common theme to these prior designs is symmetry about a center of the mill and uniformity of the cutting structure regardless of the position on the mill. While there was some intuitive rationale behind symmetry, the demands on different locations of a mill are not symmetrical and in certain cutting applications the limitations of such prior designs were made apparent.
The center of the mill has very low relative speed to the surface being cut and is a region where there is high abrasion and heat generation. In the previous designs this region tended to core badly as the matrix softened from heat and abrasion and then sloughed off to create the coring effect. As the core formed the cutting around the middle of the mill body deteriorated and as a result of that the ability of the mill to advance into the fish being milled was also impeded. The fish itself developed a peak which was the negative of the shape of the core that formed in the center bottom of the mill where the matrix was abraded off. It should be noted that in some milling applications such as when a packer with a hollow mandrel is milled there is little wear in the center of the mill as the packer mandrel is hollow. However, as the packer slips release their grip during milling the orientation of the packer can shift and the coring effect can be seen.
When chunks of the packer break off such as broken pieces of slips and the circulating fluid has to carry the cuttings to the edge of the body and then up the sides through recesses or water courses so that the cuttings can be recovered at the surface what results is high impact loading at the transition between the bottom and side of the mill such that the edge gets rounded off. This removal of the cutting structure from the periphery impedes the cutting ability of the mill. This effect can also require a trip in the hole for mill replacement which, particularly in offshore locations, can be a very expensive proposition.
The present invention focuses on tailoring the cutting structure to the nature of the expected wear on different parts of a mill. Thus the center of the mill uses a more abrasion resistant material to combat coring but the shapes of the cutting structure can be more rounded and less aggressive as most of the serious cutting occurs further away from the mill center. The outer periphery is made more impact resistant with a somewhat more aggressive cutting structure than the center of the mill. This is designed to control the rounding at the edges and associated loss of cutting structure adjacent the outer periphery. In between where the bulk of the cutting takes place the cutting structure is configured to be more robust and more highly resistant to having chunks of carbide to break off. As a result the carbide shapes in the matrix have more blunt cutting edges as opposed to the carbide at the periphery where there are longer extending segments and sharper angles so that there is a greater impact resistance with a decreased emphasis on cutting ability. These and other aspects of the present invention will be more apparent to those skilled in the art from a review of the description of the preferred embodiment and the associated figures while recognizing that the full scope of the invention is to be found in the appended claims.